An optical memory technology using an optical disc having a pit-like pattern as a high-density and large-capacity storage medium has been practically used, while expanding its application to a digital audio disc, a video disc, and a document file disc and further to a data file. Functions by which recording or reproduction of information on or from an optical disc is successfully performed with high reliability via a light beam converged to a minute spot are roughly sub-divided into a light focusing function which forms a minute spot of a diffraction limited size, a focus control (focus servo) function for an optical system, a tracking control function, and a pit signal (information signal) detecting function.
In recent years, due to the advancement of an optical system design technology and the achievement of a shorter wavelength of a semiconductor laser as a light source, the development of an optical disc having a storage capacity at a higher-than-ever density has advanced. As an approach to achieve a higher density, it is performed to increase an optical-disc-side numerical aperture (NA) in a focusing optical system which converges a light beam to a minute spot on an optical disc. At this time, an increase in the amount of aberration produced by the inclination (so-called tilt) of an optical axis presents a problem. When the numerical aperture NA is increased, the amount of aberration produced by the tilt increases. To prevent this, it is appropriate to reduce the thickness of the substrate (base member thickness) of the optical disc.
In a compact disc (CD) which might be said to be a first generation optical disc, infrared light having a wavelength λ3 (the wavelength λ3 is 780 nm to 820 nm) and an objective lens having a numerical aperture NA of 0.45 are used, and the base member thickness of the optical disc is 1.2 mm. In a second generation DVD, red light having a wavelength λ2 (the wavelength λ2 is 630 nm to 680 nm, and a standard wavelength is 650 nm) and an objective lens having a numerical aperture NA of 0.6 is used, and the base member thickness of the optical disc is 0.6 mm. In a third generation optical disc, blue light having a wavelength λ1 (the wavelength λ1 is 390 nm to 415 nm, and a standard wavelength is 405 nm) and an objective lens having a numerical aperture NA of 0.85 is used, and the base member thickness of the optical disc is 0.1 mm.
Note that, in the present specification, a base member thickness (thickness of a substrate) refers to a thickness from a surface of an optical disc (or optical information medium) on which a light beam is incident to a recording layer on which information is recorded.
Thus, the thickness of the substrate of the high-density optical disc has been reduced. In terms of economy and a space occupied by a device, an optical information device which allows information to be recorded or reproduced on or from optical discs having different base member thicknesses or recording densities is desired. To meet the desire, an optical head device including a focusing optical system capable of focusing a light beam to a spot of a diffraction limited size on each of optical discs in which substrates have different thicknesses is necessary.
As a related art example of a device which reproduces information from a CD, a DVD, and an ultra-high-density optical disc (e.g., BD (Blu-ray Disc), there is an example disclosed in Non Patent Literature 1. Using FIGS. 28 and 29, Non Patent Literature 1 will be briefly described as a first related art example.
FIG. 28 is view showing a schematic configuration of an optical head device of the first related-art example. Parallel light emitted from a blue light optical system 301 having a blue light source which emits blue light having the wavelength λ1 of 405 nm is transmitted by a beam splitter 302 and a phase plate 303 described later. The light transmitted by the phase plate 303 is focused by an objective lens 304 to irradiate an information recording layer of a first optical disc 305 (ultra-high-density optical disc) having a base member thickness of 0.1 mm. The light reflected by the first optical disc 305 follows a path reverse to an outward path to be detected by the detector of the blue light optical system 301.
Scattered light emitted from a red light optical system 306 having a red light source which emits red light having the wavelength λ2 of 650 nm is reflected by the beam splitter 302 and transmitted by the phase plate 303. The light transmitted by the phase plate 303 is focused by the objective lens 304 to irradiate an information recording layer of a second optical disc 307 (DVD) having a base member thickness of 0.6 mm. The light reflected by the second optical disc 307 follows a path reverse the outward path to be detected by the detector of the red light optical system 306.
The objective lens 304 has been designed in accordance with the base member thickness of 0.1 mm. When information is recorded or reproduced on or from a CD or DVD, a spherical aberration occurs due to the difference in base member thickness. The spherical aberration is corrected by the degree of scattering of the scattered light emitted from the blue light optical system 301 and the red light optical system 306 and the phase plate 303. When the scattered light is caused to be incident on the objective lens 304, a new spherical aberration occurs, and therefore the spherical aberration caused by the difference in base member thickness can be cancelled out by the new spherical aberration. The degree of scattering of the scattered light is set to minimize the spherical aberration. The spherical aberration cannot be completely corrected by the degree of scattering of the scattered light, and a higher-order spherical aberration (mostly fifth-order spherical aberration) remains. The fifth-order spherical aberration is corrected by the phase plate 303.
FIG. 29A is a view showing a top surface of the phase plate 303 shown in FIG. 28, and FIG. 29B is a view showing a side surface of the phase plate 303 shown in FIG. 28. When a refractive index at the wavelength λ1 is n1, the phase plate 303 is formed of a phase step 303a of a height h (h=λ1/(n1−1)) and a height 3h. The phase step 303a of the height h causes a phase difference of 1λ (λ is a wavelength in use) in light at the wavelength λ1, but does not affect a phase distribution and does not interfere with recording/reproduction on/from the optical disc 305. On the other hand, when the refractive index of the phase plate 303 at a wavelength λ2 is n2, the phase step 303a having the height 3h causes a phase difference of h/λ2×(n2−1)=0.625λ in light at the wavelength λ2. For the DVD, using the phase difference, a wavefront is converted, and the remaining fifth-order spherical aberration is corrected.
As another related-art example, a method which reproduces information using two objective lenses, which are an objective lens capable of focusing light onto an ultra-high-density optical disc and an objective lens capable of focusing light onto a CD or DVD, is disclosed in Patent Literature 1. Using FIG. 30, Patent Literature 1 will be briefly described as a second related-art example.
FIG. 30 is a view showing a schematic configuration of an optical head device of the second related-art example. A lens holder 403 includes an objective lens 401 used during recording/reproduction to/from an ultra-high-density optical disc, an objective lens 402 used during reproduction from a CD or DVD, and drive coils 404, and is suspended by wires 405 in a fixation portion 406. Magnets 407 and yokes 408 form a magnetic circuit. By allowing an electric current to flow in the drive coils 404, an electromagnetic force acts so that the objective lenses 401 and 402 are driven in a focus direction and a tracking direction. In the second related-art example, the objective lenses 401 and 402 are selectively used according to an optical disc on or from which information is recorded or reproduced.
As another method for increasing the storage capacity of an optical disc, the number of recording layers is increased. Between the recording layers, an intermediate layer needs to be provided so as to prevent the occurrence of leak-in of information. However, a spherical aberration when the thickness from the top surface of the optical disc to the recording layer thereof changes from an expected value is proportional to approximately the fourth power of the numerical aperture. Therefore, when the numerical aperture is set high, it is undesirable to thicken the intermediate layer. As a result, the leak-in of information (crosstalk) between the recording layers and interference by reflected light from each of the recording layers present a problem. One of countermeasures against the problem is disclosed in Patent Literature 2. Using FIG. 31, Patent Literature 2 will be briefly described as a third related-art example.
FIG. 31 is a view showing a schematic configuration of an optical head device of the third related-art example. FIG. 32 is a view showing a schematic configuration of an optical disc of the third related-art example. FIG. 33 is a view showing a schematic configuration of a detection hologram of the third related-art example.
The optical head device 500 includes a light source 501 which emits blue-violet laser light, a beam splitter 502, a collimator lens 503, an objective lens 504, a detection hologram 505, a detection lens 506, and a light receiving element 507 which receives laser light. An optical disc 508 includes three information recording layers. The optical head device 500 records or reproduces information on or from the optical disc 508 having the plurality of recording layers.
Using FIG. 31, a description will be given to an operation of the optical head device 500 which records or reproduces information on or from the optical disc 508. The blue-violet laser light emitted from the light source 501 is transmitted by the beam splitter 502 and converted by the collimator lens 503 into generally parallel light to be incident on the objective lens 504. The blue-violet laser light incident on the objective lens 504 is converged to a light spot onto any of the information recording layers of the optical disc 508 through a protective substrate.
The blue-violet laser light in a return path reflected by the information recording layer of the optical disc 508 follows the same optical path as followed in an outward path and is transmitted by the objective lens 504 and the collimator lens 503. The blue-violet laser light transmitted by the collimator lens 503 is reflected by the beam splitter 502, then divided by the detection hologram 505 for the detection of a servo signal, imparted with a predetermined astigmatism by the detection lens 506, and guided to the light receiving element 507. As a result, an information signal and the servo signal are generated.
A focus error signal for the optical disc 508 is generated using a so-called astigmatic method in which a focal spot imparted with an astigmatism by the detection lens 506 is detected with a quartered light receiving pattern in the light receiving element 507 or the like. A tracking error signal for the optical disc 508 is generated using a zero-order diffracted light beam and plus first-order diffracted light beams each generated by the detection hologram 505. The objective lens 504 has a numerical aperture (NA) of 0.85. The objective lens 504 is designed to be capable of forming a focal spot of a diffraction limited size onto any of the information recording layers provided in the optical disc 508 in which the thickness of a protective layer is about 0.1 mm.
As shown in FIG. 32, the optical disc 508 includes first to third information recording layers 511, 512, and 513 in which protective layers have mutually different thicknesses. Accordingly, when the focal spot is formed on, e.g., the second information recording layer 512 and information is recorded or reproduced on or from the second information recording layer 512, laser light is reflected also by each of the first and third information recording layers 511 and 513. The reflected laser light is guided to the light receiving element 507, similarly to the laser light reflected by the second information recording layer 512. The laser light reflected by each of the first and third information recording layers 511 and 513 other than the information recording layer 512 and incident on the light receiving element 507 is so-called stray light.
The detection hologram 505 has a light blocking region 505x as shown in FIG. 33. The light blocking region 505x is a circular region having a diameter D2. The light blocking region 505x is formed by, e.g., vapor depositing a metal film of aluminum or the like. The transmissivity of the light blocking region 505x is substantially zero.
FIG. 34 is a view schematically showing the optical path of the reflected light from the first information recording layer 511 when information is recorded or reproduced on or from the second information recording layer 512 of the optical disc 508 using the optical head device 500 of the third related-art example. The laser light reflected by the first information recording layer 511 has the center portion thereof blocked by the light blocking region 505x formed in the detection hologram 505 to be transmitted by the detection lens 506 and guided to the light receiving element 507. The laser light reflected from the first information recording layer 511 has light (light in the center portion thereof) including the optical axis of the laser light which is blocked by the light blocking region 505x, and does not enter a light receiving portion in the light receiving element 507.
FIG. 35 is a view schematically showing the optical path of the reflected light from the third information recording layer 513 when information is recorded or reproduced on or from the second information recording layer 512 of the optical disc 508 using the optical head device 500 of the third related-art example. The laser light reflected by the third information recording layer 513 also has light (light in the center portion thereof) including the optical axis of the laser light which is blocked by the light blocking region 505x, and does not enter the light receiving portion in the light receiving element 507.
Thus, the laser light reflected by the first information recording layer 511 and the third information recording layer 513 does not enter the light receiving portion in the light receiving element 507, and therefore does not overlap the laser light reflected by the second information recording layer 512 as the target of information recording or reproduction. As a result, fluctuations in the amount of detected laser light reflected by the second information recording layer 512 are suppressed, and stabilization of the servo signal and the information signal can be achieved.
Each of the first related-art example and the second related-art example discloses the configuration including the light sources which emit the light beams at the different wavelengths that are the red light and the blue light and having compatibility with the different types of optical discs that are the DVD and the ultra-high-density optical disc (e.g., BD). However, each of the first and second related-art examples does not disclose how to avoid leak-in of information (crosstalk) between the recording layers and interference between the reflected light beams from the individual recording layers in the case of further increasing the number of multiple layers in the ultra-high-density optical disc. On the other hand, the third related-art example discloses a means for avoiding leak-in of information (crosstalk) between the recording layers and interference between the reflected light beams from the individual recording layers, but does not disclose the configuration having compatibility with the different types of optical discs that are the DVD and the ultra-high-density optical disc (BD).
In an optical head device which records or reproduces information on or from a multilayer ultra-high-density optical disc also, it is desired to reproduce information from an existing CD and an existing DVD. However, a mere combination of the foregoing first to third related-art examples does not necessarily allow an optical head device having compatibility to be implemented. Neither of the first to third related-art examples shows a configuration for recording or reproducing information on or from a CD, a DVD, and an ultra-high-density optical disc without increasing the number of components, while ensuring performance.